PROJECT SUMMARY/ABSTRACT Biomolecular simulation is a critical tool for analysis of biopolymer structure and dynamics, investigation of intermolecular interactions, and design of new ligands and drugs. Simulation, in turn, is absolutely dependent on accurate and efficient models of the underlying structural chemistry and energetics in terms of empirical energy functions (?force fields?). Force field technology is currently in the midst of a generational transition from traditional atom-based point charges towards more intricate and accurate potentials using better electrostatic models. This proposal will continue development of the AMOEBA (Atomic Multipole Optimized Energetics for Biomolecular Applications) force field for nucleic acids (NAs), and extend the coverage of the model to naturally and synthetically modified NA components. Coupled with our 2013 AMOEBA protein parameters, the new NA force field will provide a unified model for the most important biomolecular systems. Current NA force fields lag well behind their protein counterparts in their ability to accurately model even the most typical structures under physiological conditions. The next-generation AMOEBA NA force field promises to significantly improve the fidelity and range of nucleic acids modeling. Nucleic acids are the major information carrying molecules of life. Under this research project, we will investigate several key aspects of nucleic acids, and refine the AMOEBA force field. The structures and functions of NAs are highly dependent upon the salt environment. The interplay between RNA local structural dynamics and global/tertiary folding is an intriguing question being addressed experimentally. The ability to model binding energetics, and design small molecule drugs and synthetically modified oligonucleotides will be an important growth area for future medical advances. These studies will be carried out in close collaborations with experimental colleagues. Development of an accurate and transferable next-generation force field will open up new paths toward understand and prediction of the behavior of natural and designed nucleic acid molecules. Finally, adequate sampling of large structures over longer time scales is crucial for future molecular simulations. The proposed development of high-performance, open source, parallel computer software will enable widespread application of the AMOEBA force field to nucleic acids and related biomolecular systems.